Line Of Duty: Every Series Ranked From Worst To Best

Plot: AC-12 investigate DCI Roz Huntley (Thandie Newton), who they suspect of perverting the course of justice.

With the Caddy storyline having been resolved at last, season three was very much the end of an era, but Line of Duty certainly didn't lose any momentum. Season four is a very, very strong season indeed and it speedily introduced a new big bad - the corrupt officer known only as 'H'.

Season four is another thrilling and heartfelt adrenaline rush that is ingeniously plotted and often terrifying to watch due to the crazy levels of suspense it evokes. Additionally, episode three ends with probably the best Line of Duty cliffhanger of them all.

It's not quite as good as seasons two and three though, since the revelation about ACC Derek Hilton (Paul Higgins) apparently being H is a little rushed and it has the weakest investigative target character to date.

While the other four seasons' investigative targets were all complex, sympathetic characters who, ultimately, weren't really villains, Roz Huntley is a far less likeable character and it's harder to care for her. That being said, Thandie Newton's terrific performance rescues the character in the end.

Besides, these flaws were more than compensated for thanks to great thrills, moving character beats and another explosive finale. All in all, it's a five-star season.

Best Episode: Episode six.